A heat exchanger such as a heat accumulator and a cold storage device for exchanging thermal energy between thermal transport mediums having integrated porous tubes formed in a thermal storage material is known in the art. In the heat exchanger of this kind, a heat source fluid and a hear recovery fluid flow individually in the porous tubes. One example of the heat exchanger of this kind is disclosed in Japanese Patent Laid-Open No. 2003-336974. In the heat exchanger taught by Japanese Patent Laid-Open No. 2003-336974, a plurality of flow passages are arranged in the heat storage material at regular intervals.
Specifically, in the heat exchanger taught by Japanese Patent Laid-Open No. 2003-336974, each flow passage is a porous tube flowing the heat source fluid and the hear recovery fluid individually. Those tubes are arranged between first and second tanks and individually communicated with those tanks, and the heat storage material is filled outside of the tubes. Therefore, in case a temperature of the heat storage material rises during a process of storing the heat while flowing the plurality of heat source fluids of different temperatures in the flow passages, the temperature of the heat storage material around a portion of the flow passage through which the heat source fluid of lower temperature is flowing and the temperature of the heat source fluid of lower temperature itself may be assimilated, or those temperatures may be inverted. Therefore, a heat accumulation from the heat source fluid of lower temperature to the heat storage material may be hindered.